In many people who are profoundly deaf, the reason for deafness is absence of, or destruction of, the hair cells in the cochlea which transduce acoustic signals into nerve impulses. These people are unable to derive suitable benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because there is damage to or absence of the mechanism for nerve impulses to be generated from sound in the normal manner.
It is for this purpose that prosthetic hearing implant systems have been developed. Such systems bypass the hair cells in the cochlea and directly deliver electrical stimulation to the auditory nerve fibres, thereby allowing the brain to perceive a hearing sensation resembling the natural hearing sensation normally delivered to the auditory nerve.
Prosthetic hearing implant systems have typically consisted of essentially two components, an external component commonly referred to as a processor unit and an internal implanted component commonly referred to as a receiver/stimulator unit. Traditionally, both of these components have cooperated together to provide the sound sensation to a user.
The external component has traditionally consisted of a microphone for detecting sounds, such as speech and environmental sounds, a speech processor that converts the detected sounds, particularly speech, into a coded signal, a power source such as a battery, and an external transmitter antenna coil.
The coded signal output by the speech processor is transmitted transcutaneously to the implanted receiver/stimulator unit situated within a recess of the temporal bone of the user. This transcutaneous transmission occurs via the external transmitter antenna which is positioned to communicate with an implanted receiver antenna coil provided with the receiver/stimulator unit.
This communication serves two essential purposes, firstly to transcutaneously transmit the coded sound signal and secondly to provide power to the implanted receiver/stimulator unit. Conventionally, this link has been in the form of a radio frequency (RF) link, but other such links have been proposed and implemented with varying degrees of success.
The implanted receiver/stimulator unit traditionally includes a receiver antenna coil that receives the coded signal and power from the external processor component, and a stimulator that processes the coded signal and outputs a stimulation signal to an intracochlear electrode assembly which applies the electrical stimulation directly to the auditory nerve producing a hearing sensation corresponding to the original detected sound.
As mentioned above, traditional implanted receiver/stimulator units are positioned within the head of the recipient by drilling a bed into and through the posterior section of the mastoid bone lying behind the recipient's ear. Such a bed is usually made by drilling the bone down to the lining of the brain or dura mater, so that the receiver/stimulator unit is securely held in position and does not protrude excessively past the skull surface.
The receiver/stimulator unit manufactured by the present Applicant has a package made from titanium which houses the stimulation electronics and which is fitted into a bed created in the mastoid bone. A receiver antenna coil extends from the rear end of the package and lies superficial to the bone. Other prosthetic hearing implants have included packages made from a ceramic material which are usually placed completely within the bed drilled down to the lining of the brain.
Over time it has been realized that the placing of the above packages in the mastoid bone some distance behind the ear has not always been ideal and has had some problems associated therewith. In instances where young children have been implanted with a device, it has been seen that in some recipients the package has created an external protuberance in the region of the head adjacent the implant site, which has been unsightly, intrusive, and inconvenient for the recipient. In some instances, such a protuberance can prevent the placement of a behind-the-ear processor unit over the site of the implant due to the risk of skin erosion that may result.
Further, as the package is positioned to be facing the surface of the skull, the implant package may be subject to an impact to the head in that region either directly on top of the device or as a lateral glancing blow to the device. In this regard, such devices must be designed to withstand such an impact and remain operational. In this regard, it has been found that by designing the device to have a low profile, the risk of the device sustaining a glancing, lateral blow is less likely. It is also important that the device is designed in such a manner to ensure that it is prevented from entering the cranial cavity in the event of the device being subject to an impact of excessive force.
International PCT Application No. PCT/AU00/00936 discloses an implant package capable of being located within the mastoid cavity of a recipient. This application introduces the utilization of the naturally occurring gutter lying between the sigmoid sinus, posterior osseous ear canal, the mastoid tip and the floor of the middle fossa to protect and maintain the implant package in place. This application discloses a suitably shaped implant casing capable of fitting wholly within the mastoid cavity, having a receiver coil connected thereto via flexible arms. Such a package design may have problems associated with stability of the implant within this cavity region, which could be greatly dependant upon the anatomy of the patient and the particular surgical approach used by the surgeon. Should the package be not firmly secured within the cavity, the implant may move following implantation causing tissue erosion and/or movement of the attached electrode arrays, possibly resulting in the need for re-implantation of the device.
Therefore, there is a need to provide a prosthetic hearing implant package that is capable of addressing at least some of the concerns with prior art devices.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.